96 PROCEEDINGS OF THE SOCIETY. 



Microscope when a single objective is employed, depend mainly on the 

 way in which the image-forming rays produce " parallactic shifts " in the 

 two retinal images. That was a rather complicated subject upon which 

 he would not digress at any length, but of course by " parallactic shifts " 

 he referred to the way in which on the retina of the eyes of the observer 

 points in any one plane of an object were displaced relatively to points 

 in any other plane of the object, and the manner in which this displace- 

 ment differed for the two eyes. 



(Mr. Rheinberg here demonstrated his point on the blackboard.) 



These matters, it appeared to him, might also have an interesting 

 bearing on the question which Mr. Beck brought forward as to the best 

 arrangement of the tubes, namely, as to whether they should be parallel 

 or whether they should be converging at a certain angle, and he thought 

 that was a very interesting point which had not yet been thoroughly 

 cleared up and would repay some more careful work. He would like to 

 congratulate the authors of these two papers on the extremely interesting 

 instruments they had brought to the notice of the Society.* 



Mr. Cheshire said that it seemed to him, so far as he understood 

 Dr. Jentzsch's Paper, that he had made the matter of the stereoscopic 

 relief perfectly plain. He was inclined to think that Mr. Beck and 

 Mr. Rheinberg had not quite understood the paper. He thought that 

 Dr. Jentzsch said quite clearly that when his Microscope was used with 

 an interocular distance equal to that of the observer, there was no 

 true stereoscopic effect, but that if the interocular distance of the instru- 

 ment were set wrongly, then one obtained either orthoscopic or pseudo- 

 stereoscopic effects according as the interocular adjustment of the 

 instrument was less or greater than the interpupillary distance of the 

 observer. 



In regard to the paper generally, he thought that the Society was 

 to be congratulated on such an accession to its records — it was an epoch- 

 marking paper, and especially so from a side which had not been touched 

 upon, and that was from the psychological side. 



There was one other point he would like to mention. Dr. Jentzsch 

 in his paper had said that the upper focal-plane of the objective was 

 the position in which division took place ; Mr. Beck on the other hand 

 had disagreed, saying that it should take place in the principal plane. 

 He personally thought it should be in the focal plane, because division 

 took place as though in the Ramsden disk which was the exit-pupil of 

 the system, the entrance-pupil being the diaphragm in the lower 

 focal plane of the condenser. 



What had been said in regard to the arrangement of the prisms was 

 interesting because the explanation of the phenomena was given as 

 far back as the year 173S in Smith's Optics. [Here Mr. Cheshire de- 

 monstrated his meaning on the blackboard by an illustration of a pair of 

 eyes looking at two points, one at infinity and the other at a fixed point] 

 It was a very remarkable thing that Smith in 1738 said that the phe- 



* Mr. Rheinberg writes to say that since the above remarks were made he has 

 learnt from a competent observer that the same stereoscopic effect with high 

 powers had been observed under certain conditions, when using an Abbe stereo- 

 scopic eye-piece. 



